In mobile communications systems and in packet communications systems utilizing IP, advancement in the rate of digital signal processing by DSPs (Digital Signal Processors) and enhancement of bandwidth have been making possible high bit rate transmissions. If the transmission rate continues increasing, bandwidth for transmitting a plurality of channels can be secured (i.e. wideband), so that, even in speech communications where monophonic technologies are popular, communications based on stereophonic technologies (i.e. stereo communications) is anticipated to become more popular. In wideband stereophonic communications, more natural sound environment-related information can be encoded, which, when played on headphones and speakers, evokes spatial images the listener is able to perceive.
As a technology for encoding spatial information included in stereo audio signals, there is binaural cue coding (BCC). In binaural cue coding, the coding end encodes a monaural signal that is generated by synthesizing a plurality of channel signals constituting a stereo audio signal, and calculates and encodes the cues between the channel signals (i.e. inter-channel cues). Inter-channel cues refer to side information that is used to predict channel signal from a monaural signal, including inter-channel level difference (ILD), inter-channel time difference (ITD) and inter-channel correlation (ICC). The decoding end decodes the coding parameters of a monaural signal and acquires a decoded monaural signal, generates a reverberant signal of the decoded monaural signal, and reconstructs stereo audio signals using the decoded monaural signal, its reverberant signal and inter-channel cues.
Thus, non-patent document 1 and non-patent document 2 are presented as examples disclosing techniques of encoding spatial information included in stereo audio signals. FIG. 1 is a block diagram showing primary configurations in stereo audio coding apparatus 100 disclosed in non-patent document 1. Referring to FIG. 1, monaural signal generating section 11 generates a monaural signal (M) using the L channel signal and R channel signal constituting a stereo audio signal received as input, and outputs the monaural signal (M) generated, to monaural signal coding section 12. Monaural signal coding section 12 generates monaural signal coded parameters by encoding the monaural signal generated in monaural signal generation section 11, and outputs the monaural signal coded parameters to multiplexing section 14. Inter-channel cue calculation section 13 calculates the inter-channel cues between the L channel signal and R channel signal received as input, including ILD, ITD and ICC, and outputs the inter-channel cues to multiplexing section 14. Multiplexing section 14 multiplexes the monaural signal coded parameters received as input from monaural signal coding section 12 and the inter-channel cues received as input from inter-channel cue calculation section 13, and outputs the resulting bit stream to stereo audio decoding apparatus 20.
FIG. 2 is a block diagram showing primary configurations in stereo audio decoding apparatus 20 disclosed in non-patent document 1. Referring to FIG. 2, separation section 21 performs separation processing with respect to a bit stream that is transmitted from stereo audio coding apparatus 10, outputs the monaural signal coded parameters acquired, to monaural signal decoding section 22, and outputs the inter-channel cues acquired, to first cue synthesis section 24 and second cue synthesis section 25. Monaural signal decoding section 22 performs decoding processing using the monaural signal coded parameters received as input from separation section 21, and outputs the decoded monaural signal acquired, to allpass filter 23, first cue synthesis section 24 and second cue synthesis section 25. Allpass filter 23 delays the decoded monaural signal received as input from monaural signal decoding section 22 by a predetermined period, and outputs the monaural reverberant signal (MRev′) generated, to first cue synthesis section 24 and second cue synthesis section 25. First cue synthesis section 24 performs decoding processing using the inter-channel cues received as input from separation section 21, the decoded monaural signal received as input from monaural signal decoding section 22 and the monaural reverberant signal received as input from allpass filter 23, and outputs the decoded L channel signal (L′) acquired. Second cue synthesis section 25 performs decoding processing using the inter-channel cues received as input from separation section 21, the decoded monaural signal received as input from monaural signal decoding section 22 and the monaural reverberant signal received as input from allpass filter 23, and outputs the decoded R channel signal (R′) acquired.
Now, conventional mobile telephones already feature multimedia players with stereo functions and FM radio functions. In addition to this, fourth-generation mobile telephones and IP telephones are anticipated to have additional functions for recording and playing stereo speech signals.
Non-Patent Document 1: ISO/IEC 14496-3: 2005 Part 3 Audio, 8.6.4 Parametric stereo
Non-Patent Document 2: ISO/IEC 23003-1: 2006/FCD MPEG Surround (ISO/IEC 23003-1: 2007 Part1 MPEG Surround)